I have a qustion about antenna loading coils. Most of the loading coils I have seen for part 15 AM antennae are air-wound coils. Has anyone ever tried using a ferrite core to increase the amount of inductance per turn and thus reduce the amount of turns (and wire) required? After all ohmic losses in the coil are one of the major factors reducing antenna efficiency. And at 100mw of power, the core would not saturate.
Speaking of the loading coils, why do they always seem oriented with their axis vertical? Some current must flow thru the coil that could contribute to radiation. If you look at the ferrite rod antenna coil in an AM portable, it is always oriented so that the axis is horizontal, not vertical. If you rotate it so that it is vertical, the signal disappears. It would seem to me that if you orient the loading coil horizontally instead of vertically, so that it is in the same plane as the antenna coil in a receiver, you might get a bit more useful radiation, especially if the loading coil has a large diameter. Any thoughts on this?
You raise some interesting questions. I'm no expert but here are some thoughts:
Has anyone ever tried using a ferrite core to increase the amount of inductance per turn and thus reduce the amount of turns (and wire) required?
Yes, this is done quite a lot. The SSTRAN uses mini-inductors as loading coils with its wire antenna. The Rangemaster (R) uses a wound torroid. It does, indeed, reduce the wire and the size, but the core materials dissipate power more than air. This lowers the Q. In part 15 AM we are limited to the amount of power we can produce so it becomes a design tradeoff between physical size and power loss in the coil. Though every milliwatt counts, the major resistive loss in a transmitting system is in the ground circuit so some increased loss in the coil can be tolerated with no practical effect. With ham and two-way handhelds with short antennas where the power is not restricted the loading coil loss is not much of a factor since it can be made up with increased power.
Speaking of the loading coils, why do they always seem oriented with their axis vertical?
Usually this is a more convenient way to mount them physically. Electrically, it makes little difference since their length is such a small fraction of the wavelength and very little power is radiated from the coil.
If you look at the ferrite rod antenna coil in an AM portable, it is always oriented so that the axis is horizontal, not vertical. If you rotate it so that it is vertical, the signal disappears. It would seem to me that if you orient the loading coil horizontally instead of vertically, so that it is in the same plane as the antenna coil in a receiver, you might get a bit more useful radiation, especially if the loading coil has a large diameter. Any thoughts on this?
Good and accurate observation. AM broadcast waves are polarized so the electric field is vertical and the magnetic field is horizontal. The ferrite loop ant. in a radio responds to the magnetic field so the ferrite is oriented horizontally to capture the maximum possible magnetic field. The field needs to cut across the maximum number of wires on the antenna coil. This is also why they are directional if rotated about a vertical axis. In a car radio, the antenna responds to the vertical electric field so they are vertical. Loop antennas of this small size (compared to wavelength) do not radiate efficiently which is why a wire or whip is preferred. Even the wire or whip is not very efficient because the physical length of the wire is a very small fraction of the wavelength.
This is just an overview of answers to your questions but I hope it is useful regardless. If you want to dig into any of this further, just ask.
Neil
This pic shows the triangular coil form made from corks, dowel rods, tooth picks and pop-sickle sticks. This will be indoors to connect AMT3000 to a Wintenna (window antenna).
Good frame design. Not sure how its shape would differ from the standard circular shaped coil in respect to loading characteristics, or if the frame structure itself would affect the coil's properties...which it probably will.
The wood of that frame for the coil most likely would affect the coil's properties much like how a nearby tree limb swaying in the wind would affect a coil or antenna's resonant properties.
And there is the consideration of the wood changing its properties by temperature, moisture and such.
To me it looks more like a mini-tower than a coil form! Just make sure you put on your climber's belt first before attempting to replace the beacon!
Pretty cool though Carl.
RFB